Loneliness is an unpleasant feeling that arises due to a discrepancy between the desired and achieved quality of ones social network (Peplau &amp; Perlman, 1982), and has a deleterious effect on various aspects of psychological well-being (Heinrich &amp; Gullone, 2006). Loneliness is particularly relevant to investigate in adolescence because ones social network changes substantially during this period (e.g., increased need for intimate peer relations; Parkhurst &amp; Hopmeyer, 1999). To date, however, there is a dearth of longitudinal studies on loneliness in adolescence and the transition to adulthood. Therefore, the present dissertation examined the course of loneliness trough mid- and late adolescence, its developmental antecedents, psychological consequences, and the mechanisms underlying these associations. Eight studies, summarized in six empirical chapters, have been conducted to address this general aim. Peer- and self-reported questionnaire data from four longitudinal datasets were used. These six empirical chapters are preceded by a general introduction and followed by a general discussionAn in-depth investigation of the longitudinal associations between loneliness and two of its strongest correlates  depressive symptoms and self-esteem  was conducted. Regarding the association between loneliness and depressive symptoms, cross-lagged results pointed to a transactional model with predominant influences from loneliness to depressive symptoms. This predominant path was partly mediated by passive coping strategies in general, and uncontrollable ruminative thoughts in particular. Active coping strategies, however, could not prevent lonely adolescents from developing depressive symptoms. Moreover, moderation by the personality trait neuroticism was found, indicating that adolescents high in neuroticism were particularly at risk to get stuck in the vicious circle between loneliness and depressive symptoms. Regarding the association between loneliness and self-esteem, a transactional model was uncovered in which low self-esteem and loneliness reciprocally affected one another across time. Moreover, perceived (i.e., self-reported) social acceptance was a partial mediator in this process, whereas actual (i.e., peer-reported) social acceptance was not. As these results on the association between loneliness and psychological well-being evidenced important inter-personal and intra-personal correlates of loneliness, we further examined the joint effects of both types of correlates as predictors of adolescent loneliness. Specifically, additive, mediating, and moderating effects between several inter- and intra-personal predictors of loneliness were examined. The four investigated inter-personal factors (i.e., actual social acceptance, victimization, friendship quantity, and friendship quality) and two intra-personal factors (i.e., shyness and self-esteem) were found to be unique predictors of loneliness. Moreover, important interplay between inter- and intra-personal factors in predicting loneliness was revealed. Finally, the developmental course of loneliness through mid- and late adolescence was investigated, both at the group level and at the level of subgroups. A general decreasing trend in loneliness was evidenced. However, substantial individual differences were observed, as we identified five loneliness trajectories (i.e., stable low, low increasing, moderate decreasing, high decreasing, and chronically high). These five trajectory classes were differentially predicted by personality traits (i.e., extraversion, agreeableness, and emotional stability) and had different psychosocial outcomes (i.e., depressive symptoms, self-esteem, anxiety, and perceived stress), providing evidence for the validity of the different loneliness trajectories.In sum, the present dissertation identified important antecedents, consequences, and underlying mechanisms in the development of loneliness across mid- and late adolescence, and therefore, addressed an important gap in the loneliness literature. Moreover, the results of the present dissertation raise new questions that stimulate future research and, hence, contribute to the continuing development of loneliness research and theory.